One example of a normal semiconductor power switching element is an IGBT (Insulated Gate Bipolar Transistor). One application example of the semiconductor power switching element is a control circuit for use in power electronics control, such as an inverter circuit which controls a three-phase motor.
FIG. 8 is a circuit diagram schematically showing this conventional inverter circuit. As shown in FIG. 8, the conventional inverter circuit (three-phase circuit here) includes circuits (hereinafter referred to as “phase switching circuit”) 23 each formed by connecting a switch function portion (hereinafter referred to as “upper arm”) 23H and a switch function portion (hereinafter referred to as “lower arm”) 23L in series, and the number of circuits corresponds to the number of phases (three here). Each of the upper arm 23H and the lower arm 23L is constituted of a switching element 21 and a diode 22 which are connected in parallel to each other. The switching element 21 is constituted of, for example, the IGBT using silicon. The upper arm 23H is connected to a high potential wiring 25, and the lower arm 23L is connected to an earth potential wiring 24. Midpoints 26 between the arms 23 are connected to input terminals (hereinafter referred to as “motor input terminals”) 27 of a three phase AC motor that is a load. The potential of the midpoint 26 can be controlled by adjusting ON-OFF timings of the upper arm 23H and the lower arm 23L. To be specific, the potential of the midpoint 26, that is, the potential of the input terminal 27 becomes equal to the earth potential 24 when the lower arm 23L is ON and the upper arm 23H is OFF. Meanwhile, the potential of the midpoint 26, that is, the potential of the input terminal 27 becomes equal to the high potential 25 when the upper arm 23H is ON and the lower arm 23L is OFF. Thus, the three-phase motor 28 can be controlled by switching the potential of the motor input terminal 27 between the earth potential 24 and the high potential 25.
However, the response speed of the switching element 21 and the response speed of the diode 22 are limited. Therefore, even if a signal for switching from an ON state to an OFF state is supplied to the switching element 21 and the diode 22, the switching element 21 and the diode 22 do not become the OFF state immediately. On this account, in a case where the ON-OFF switching of the upper arm 23H and the ON-OFF switching of the lower arm 23L are carried out at the same time, both the upper arm 23H and the lower arm 23L may become the ON state. Such a state is a state where the high potential 25 and the earth potential 24 are short-circuited, so that a large current flows to the inverter circuit. Moreover, since this current becomes a loss current, the switching loss increases, and the power use efficiency decreases. In the inverter circuit, high efficient inverter control is carried out by high speed switching. Therefore, the switching loss is repeated for the number of switchings. Thus the entire switching loss becomes large. On this account, conventionally, the timing of the switching is determined in consideration of the response speed of the switching element 21 and the response speed of the diode 22. In other words, the frequency of the inverter control is determined in accordance with the limitations of the response speed of the switching element 21 and the response speed of the diode 22. However, in the case of carrying out the high efficient inverter control by further higher speed switching, further increase in speed of the switching of the switching element 21 and the diode 22 is required.
However, in the case of using the IGBT as the switching element, since the IGBT is a bipolar device, the lifetime of the minority carrier is long, and the time required for the reverse recovery is long. Therefore, the switching from ON to OFF is not carried out at high speed. So, a MOSFET (metal-oxide semiconductor field-effect transistor) that is a unipolar device is used as the switching element. Since the unipolar device is not affected by the minority carrier, the switching from ON to OFF can be carried out at high speed. However, in the case of the MOSFET made of silicon, the ON-resistance Ron (Ωcm2) per unit area is high, and a conduction loss due to heat generation increases.
In contrast, one example of a diode whose switching is increased in speed is a fast recovery diode which is subjected to carrier lifetime control. However, it is difficult for the fast recovery diode to operate at a high frequency of several tens of kHz or more. Moreover, the fast recovery diode is the bipolar device. Therefore, although the ON-resistance decreases due to the diffusion of the minority carriers, the lifetime of the minority carrier is long. On this account, the switching from ON to OFF takes time. One example of a diode whose switching is further increased in speed is a schottky diode in which a schottky electrode schottky-contacts a semiconductor. Since the schottky diode is the unipolar device and is not affected by the minority carrier, the switching from ON to OFF can be carried out at high speed. However, since the schottky diode made of silicon has the withstand voltage of only about 100 V, it cannot be used in a power electronics field which requires the withstand voltage of 600 V or higher.
Moreover, since the IGBT and diode made of silicon are subjected to the carrier lifetime control, they cannot be integrated in one chip.
Here, proposed is to form the switching element and the diode, used in the inverter circuit, etc., by wide band-gap semiconductors.
For example, regarding the diode, the schottky diode made of the wide band-gap semiconductor has the withstand voltage of 600 V or higher, has the ON-resistance which is adequately lower than that of the diode made of silicon, and can carry out the switching from ON to OFF at high speed.
Also, regarding the switching element, the MOSFET made of the wide band-gap semiconductor has the ON-resistance per unit area which is adequately lower than that of the IGBT made of silicon, can secure the withstand voltage, and can carry out the switching from ON to OFF at high speed.
However, even in the case of a SiC-MISFET, by a parasitic diode constituted of a PN junction of a p-type region and an n-type region in a semiconductor device, a reverse recovery time delay may occur in the case of switching from an ON state of the parasitic diode to an OFF state of the SiC-MISFET when a reverse bias is applied.
For example, when a positive voltage that is a counter electromotive voltage generated by an inductance load when the switching element is turned OFF is applied to a source electrode, positive holes as minority carriers are implanted in the n-type region via the parasitic diode, and this causes the reverse recovery time delay of the operation of the parasitic diode.
Meanwhile, the schottky diode and the MOSFET that is the switching element can be integrated in one chip in such a manner that a vertical MOSFET is made of the wide band-gap semiconductor, and the schottky electrode is disposed so as to form a schottky junction with a drift region of the vertical MOSFET (see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Publication 2002-203967